This invention relates to an improved process for the conversion of aromatic hydrocarbons. More specifically, the invention concerns disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,097,543 (Haag et al.) teaches toluene disproportionation for the selective production of paraxylene using a zeolite having a silica/alumina ratio of at least 12 and a constraint index of 1 to 12, which zeolite has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,276,437 (Chu) teaches transalkylation and disproportionation of alkylaromatics to yield predominantly the 1,4-alkylaromatic isomer using a zeolite which has been modified by treatment with a compound of a Group IIIB element. The catalyst optionally contains phosphorus, and it is contemplated that the Group IIIB metal is present in the oxidized state.
U.S. Pat. No. 4,922,055 (Chu) teaches toluene disproportionation using a zeolite, preferably ZSM-5, containing framework gallium, shown to be superior to non-framework gallium. Selective production of paraxylene is not disclosed in this reference.
The accepted mechanism for transalkylation and disproportionation is believed to be effected via a strong Brxc3x6nsted acid such as is provided by a zeolitic aluminosilicate. A lower-energy path, however, would provide potential for greater selectivity and improved economics.
U.S. Pat. No. 4,724,066 (Kirker et al.) describes a catalyst for toluene disproportionation containing a zeolite such as ZSM-5 or Beta and a microcrystalline, microporous aluminum phosphate component. The reference describes incorporation of optional metal components described as a xe2x80x9cdifficultly reducible oxidexe2x80x9d or an optional hydrogenation component. Both optional components may be chosen from a very long list of elements, with the hydrogenation component list including rhenium.
It is an object of the present invention to provide an improved catalyst for the disproportionation and/or transalkylation of aromatic hydrocarbons to yield desirable alkylaromatic isomers. A specific objective is obtain a high yield of paraxylene by disproportionation of toluene, or transalkylation of toluene and higher aromatics.
This invention is based on the discovery that high activity with potential for selectivity to paraxylene is obtained by disproportionation of toluene using a zeolitic catalyst containing a reduced weak metal.
A broad embodiment of the present invention is directed to a process for the disproportionation of a toluene feedstock to obtain a product comprising paraxylene using a catalyst comprising a molecular sieve having a pore diameter of from about 5 to 8 xc3x85, an elective refractory inorganic oxide and a reduced weak metal. Optionally, the feedstock comprises C9 aromatics which are disproportionated to yield additional C8 aromatics. The catalyst preferably is subjected to a conditioning step prior to its use for disproportionation/transalkylation in order to deposit a controlled concentration of carbon on the catalyst and increase its selectivity to paraxylene in the product.
The catalyst of the present invention comprises a zeolitic aluminosilicate preferably selected from MFI, MEL, MTW and TON, and most preferably comprises MFI. The optional refractory inorganic oxide preferably comprises one or both of alumina and silica, and an amorphous aluminum phosphate binder is favored. The catalyst also contains a non-framework weak metal. The weak metal is defined by its ability to effect H2/D2 exchange without effecting methylcyclohexane dehydrogenation at 300xc2x0-500xc2x0 C. The metal is preferably selected from the group consisting of Ga, Re and Bi and optimally is selected from gallium and bismuth.
A process combination employing the subject process optionally comprises a xylene-separation zone in which paraxylene is recovered by adsorption.
These, as well as other objects and embodiments, will become apparent from the detailed description of the invention.